Highly Luminous N-Substituted LiMSiON:Eu (M = Ca, Sr, and Ba) for White NUV Light-Emitting Diodes

The N-substituted LiMSiO:Eu (M = Ca, Sr, and Ba) phosphors were systematically prepared and analyzed. Secondary-ion mass spectroscopy measurements revealed that the average N contents are 0.003 for Ca, 0.009 for Sr, and 0.032 for Ba. Furthermore, the N incorporation in the host lattices was corroborated by infrared and X-ray photoelectron spectroscopies. From the photoluminescence spectra of LiMSiO:Eu (M = Ca, Sr, and Ba) phosphors before and after N doping, it was verified that the enhanced emission intensity of the phosphors is most likely due to the N doping. In LiMSiO:Eu (M = Ca, Sr, and Ba) phosphors, the maximum wavelengths of the emission band were red-shifted in the order Ca < Ba < Sr, which is not consistent with the trend of crystal field splitting: Ba < Sr < Ca. This discrepancy was clearly explained by electron-electron repulsions among polyhedra, LiO-MO , SiO-MO , and MO -M'O associated with structural difference in the host lattices. Therefore, the energy levels associated with the 4f5d energy levels of Eu are definitely established in the following order: LiCaSiO:Eu > LiBaSiO:Eu > LiSrSiO:Eu. Furthermore, using the Williamson-Hall (W-H) method, the determined structural strains of LiMSiO:Eu (M = Ca, Sr, and Ba) phosphors revealed that the increased compressive strain after N doping induces the enhanced emission intensity of these phosphors. White light-emitting diodes made by three N-doped phosphors and a 365 nm emitting InGaN chip showed the (0.333, 0.373) color coordinate and high color-rendering index ( = 83). These phosphor materials may provide a platform for development of new efficient phosphors in solid-state lighting field.